Accurate knowledge of the likelihood of rockfalls (or worse tunnel collapse) and the proximity of cavities and inclusions behind the rockface gives considerable benefits.

• Allows planning of mining operations to reduce lost production.
• Protects the investment in the mining operation, people and equipment.
• Gives an accurate picture of the seismic activity around the mine workings for forward planning of activities

A monitoring operation which does not need to be installed in the mine means

• It can be set up and operating within a few hours. 
• It can produce meaningful data for decisions within one working day.
• Useful geomechanical parameters can be derived from the smallest amount of recorded data.
• It does not need intrinsic safety clearance, long cables or skilled installation in the mine
• Much lower fully installed cost than alternative equipment

Hitherto the monitoring of seismic activity around deep mining operations has encountered a number of significant problems.

Installation of equipment in a mine is time consuming and needs sanction for use. Lack of adequate dynamic range of the monitoring equipment used to date has meant that some seismic activity signals have been swamped by noise generated by normal mining operations

Work done by A. Bryan-Jones, P. Styles & S.Toon, of the Microseismology Research Group, University of Liverpool, England, in association with Magus Electronics, has resulted in equipment which overcomes these problems and which has been tried and tested in deep mining operations in England.

The key features of the equipment :

• Wide dynamic range (138dB    freq. ½ Hz to 1 kHz).
• 24 bit ADC resolution down to 216 nanovolts at input.
• Advanced signal correlation techniques.

Provide a high sensitivity to capture microseismic activity and also give correct, accurate operation even if the signal  is swamped by noise generated by normal mining.

The following is a description of its use in practice.

Consider the deep mine operation shown opposite. This has a longwall coal face 500 metres below the surface. The main seam lies below a Dolerite sill with layers of Sherwood Sandstone, Mercia Mudstone, Middle and Lower Lias then Boulderclay to the surface.

Mining excavations cause changes in the stress field in the surrounding rock mass, which in turn cause seismic events.

Most seismicity is expected above and behind the working face i.e. in the caving zone.

 

Rock mechanics models predict how the rock mass behaves as a result of changes in the stress field.

The rock mechanics model is shown below. The area of excess vertical shear stress is clearly shown in the sand stone above the dolomite sill, also the Yield Zone and Bed Separation Zone below the sill.

Recording Equipment used 9 three-component seismometers down three boreholes (1:4:4) at various depths.
The data was recorded digitally on ‘Vibrosound’ machines using  24-bit ADC at 1kHz sampling rate

1.5 seconds of data were recorded for each event, and stored on a 20MB Flashcard (could hold data from 600 events).
The best position for the seismometers is about halfway down the boreholes as a good compromise between best sensitivity (bottom) and best coverage (top). The recording unit was on the surface .

Nearly all events were recorded at only one seismometer. Each event is recorded as a data set of event time (to +/- 1 mSec), displacement, acceleration, velocity, frequency, then analysed into magnitudes in 3 orthogonal directions

Events are located by finding the direction of polarisation of the P-wave to give the source-receiver direction. The difference between P-wave and S-wave arrival times gives the distance to the source from the receiver.     This in fact gives two locations in opposite directions, so an assumption is made to remove one of the locations e.g. by assuming that events cannot occur above the ground!

The locations of events can be displayed 3-dimensionally as shown. One can immediately see if there is cause for concern. A random scatter would indicate normal microseismic activity.
However if there is a cluster of events as shown here, this is indicative of excessive stress in the rock structure at that location.
A further refinement of the data is a 

3 - dimensional display where each event is depicted as seismic moment (colour) and source radius, magnitude  and velocity in 3 directions (ellipse).
In the case of the longwall coalface , there is virtually no recorded microseismic activity at seam level.
Most activity occurs about 150m above the seam i.e. in and above the sandstone. This agrees to a good extent with the rock mechanics model. The stress levels and clustering is indicative of a major cause for concern.

Source Mechanisms. It is virtually impossible to quickly find a unique double couple source mechanism, even more so a moment tensor source mechanism.

Using co-ordinate transform and grid search techniques, a (possibly non-unique) strike, rake and dip can be quickly calculated for most events.

Location of oil or water deposits or of large cavities comes from the seismic signals generated by echoes from the boundaries of the deposits or cavities. Usually these signals are at a low signal level and are normally swamped by background noise. The sensitivity and wide dynamic range of the equipment allows these signals to be captured and identified.

The Vibrosound SP1 Seismic Vibration Monitor has wide dynamic range (138 dB 1/2 Hz to 1 kHz), to prevent saturation of the signal recording  channel in noisy environments. Its 24 bit ADC resolution, giving a sensitivity down to 216 nanovolts at inputs,  together with advanced signal correlation techniques, means that seismic events can faithfully be captured at a considerable distance.The use of existing boreholes makes for easy installation and set up User selectable trigger levels, event length, sampling rate, data storage and transducer options means the equipment can be set up and configured to give the optimum monitoring and recording system to match the characteristics of the microseismic activity.

We have software packages for importing and converting the captured data into your site geology computer model.

For further, comprehensive, details of the equipment, its use, casing, computer interface, software packages, transducers, printer, plotter and other options  please contact us by e-mail using the link below or at the address given in the About Us page.

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